Evaluation of transmission quality is an important aspect of fiber optic communications systems. Prior art evaluation of transmission quality is performed by electronic detection where the detected sequence of digital information is compared using a functional relationship to the actual value transmitted along with the information such as parity checks or error correction coding. However, the detection of errors does not provide an indication of the origin or cause of the transmission error. Many factors can produce transmission factors including limited received power, chromatic dispersion effects, poor optical signal-to-noise ratio, polarization mode dispersion (PMD) and nonlinear effects. The issue of PMD is of particular interest as it is expected that PMD will become the major source of error for optical networks transmitting information at data rates greater than 20 Gbits/s. Hence, it is important to measure PMD and determine the impact of PMD or PMD impairment on individual dense wavelength-division multiplexing (DWDM) channels. It is important to distinguish between PMD and PMD impairment. The PMD describes the birefringence of the optical link while the PMD impairment describes the effect of that birefringence on a DWDM channel or frequency band. Even large PMD may not cause PMD impairment if all optical frequencies comprising a frequency band propagate throughout the link in predominantly the same polarization state.
PMD refers to the temporal pulse distortion that arises from different propagation speeds for light of differing polarization states through an optical medium such as a single mode optical fiber. PMD arises from the birefringence in an optical fiber that increases with fiber length. The larger the birefringence, the larger the PMD and the more rapidly the polarization state changes with wavelength and with fiber length. Hence, a typical method of determining PMD involves analyzing the evolution of the polarization state with wavelength. The PMD induced delay is defined as:                     τ        =                  Δθ                      2            ⁢            πΔ            ⁢                                                   ⁢            v                                              (        1        )            where Δθ is the rotation angle on a Poincare sphere and Δv is the optical frequency span that produced Δθ. To determine PMD in an operational network requires that the polarization state analysis be performed over the width of a single channel or frequency band of the DWDM system carrying data. Thus, spectral width is related to the frequency band spacing. The present International Telecommunications Union (ITU) grid is placed at 100 GHz or 0.8 nm with further reduction of frequency band spacing being planned. This requires that the polarization state measurements are performed with high spectral selectivity.
Westbrook et al., in “Wavelength sensitive polarimeter for multichannel polarization and PMD monitoring,” OFC 2002, pp. 257-259, have disclosed a wavelength selective polarimeter that is based on fiber grating technology. The disadvantage of this approach is that the current grating technology is limited to a resolution of about 0.01 nm. Roudas et al., in “Coherent heterodyne frequency-selective polarimeter for error signal generation in higher-order PMD compensators,” OFC 2002, pp. 299-301, disclosed a heterodyne polarimeter based on Stokes vector measurements that requires sequential switching of the local oscillator (LO) polarization state. The heterodyne polarimeter potentially offers high resolution but the technique disclosed by Roudas et al. resembles that of classical intensity based polarimeters and does not take advantage of the phase information provided by the heterodyne signal. Therefore, sequential switching of the polarization state is required. This may lead to erroneous measurements in systems where the polarization state is time dependent.